1. Field of the Invention
The present invention relates to a semiconductor laser drive control device for driving a semiconductor laser based on input image data.
2. Description of the Related Art
As high-speed and high-quality image forming apparatus, a copying machine, a laser beam printer, and the like employing an electrophotography system have conventionally been known. Recently, digital contents have been growing popular both at home and at the office, and accordingly a demand for improving image quality in the image forming apparatus has further been increasing and a recording resolution of the apparatus has also been making progress toward a higher density, such as 1200 dpi, 2400 dpi, or 3600 dpi.
In such image forming apparatus based on the electrophotography system, binary or multivalued data are converted into an amount of irradiating light onto an image carrier. Then, a surface of the image carrier is scanned with, for example, a laser beam having the converted amount of irradiating light. Such processing enables any images including from a binary image such as a character to a halftone image such as a photograph to be formed.
Examples of a method for reproducing the halftone include methods such as a dither method, an error diffusion method, and a density pattern method (a dot pattern method). The use of such method enables each pixel to be output in a multivalued form, whereby an excellent output image can be provided.
As a method for converting the output multivalued data into an amount of irradiating light, a Pulse Width Modulation (PWM) method and a Power Modulation (PM) method have been proposed. Also, a method for controlling an amount of light, which is a combination of the above methods, has been proposed.
The image forming apparatus has recently been requiring a higher density in a recording resolution and higher image quality. Accordingly, challenges have arisen to provide excellent dot reproducibility in a highlight region and to reproduce a high-density dot and a high-density line such as an outline character and an outline line.
In order to fulfill the requirement for the higher density in a recording resolution, an increase in speed of a laser driver device is a must; however, the increase in speed causes the following negative effect. That is, an amount of light is decreased due to a defective rising of a laser emission in a highlight region, whereas an amount of light is increased due to a defective falling of the laser emission in a high-density region, and therefore linearity of the laser emission is reduced, whereby image quality is deteriorated.
Therefore, providing excellent dot reproducibility in a highlight region has been proposed, on which a technique has been known as that described in Japanese Patent No. 2698099, or Japanese Patent Application Laid-open Nos. 9-116750 (1997), 2001-130050, 2002-361922, 2002-361925, 2003-266763, 2006-96008, 2000-177171, 6-155800 (1994), or 2004-122587.
Alternatively, a correction technique using an LUT (look-up table) in an engine capable of a multivalued output within one pixel has also been widely known.
However, it has not been able to correct collapse of the resolution due to a defective falling of a laser optical signal in a high-density region where a signal of the laser optical signal changes from an intermittent mode to a continuous mode.
In the case of carrying out a correction in a specific region comprised of only a highlight region or high-density region, because an appropriate correction over all gradations has not been possible, the gradations have become discontinuous between correction application and non-application regions, and a tone jump has been likely to occur.
Also, in the case of carrying out a correction with a same correction amount over all gradations, an effective correction has not been possible from a highlight region where a turn-on delay of a laser causes a problem to a high-density region where a turn-off delay of the laser causes a problem.
On the other hand, regarding a correction technique using an LUT in the engine capable of a multivalued output within one pixel, a satisfactory correction has not been possible in the case of a smaller number of bits per one pixel, due to a recent increase in resolution. Specifically, under the condition of a resolution of 1200 dpi, 2400 dpi, or the like and an output of 1 to 4 bits, a satisfactory correction has not been possible.
Also, the LUT to be used for the correction is dependent on a pixel condition around a pixel to be corrected, in particular, on pixel distance between a falling of a pixel preceding the pixel to be corrected in a scanning direction and a rising of the pixel to be corrected. For this reason, a simple LUT correction within one pixel has not been useful for a satisfactory correction, whereby a problem has arisen in reproducibility of gradations.
On the other hand, there also exists a problem on a technology for correcting nonlinearity of the electrophotography. Recent electrophotographic image recorder is in so-called digital form, wherein an image pattern is formed by laser blinking and a halftone region is expressed based on a ratio of an area receiving the laser blinking. Also, a conventional analog system has a similar problem in development characteristics. That is, a highlight region has a low image density due to the non-adhesion of toner and a high-density region exhibits S-characteristics due to the saturation of toner concentration. This is generally known as fundamental nonlinearity arising during latent image processing to development processing in the electrophotography.
In order to correct such basic characteristics present in the electrophotography, a correction inverse of the S-characteristics can be carried out such that an amount of light in the highlight region is enhanced and an amount of light in the high-density region is reduced. As such a technique, a technique providing excellent gradation characteristics has been disclosed in Japanese Patent Application Laid-open Nos. 2000-177171 and 6-155800 (1994). However, the disclosed technique is intended to carry out a correction such that an amount of light is enhanced and reduced for one-dot/one-space images and one-line/one-space images respectively. Therefore, an image region where a turn-off delay of a laser occurs has been unable to be an object to be corrected.